FIG. 8 is a rear view showing a conventional portable telephone set, and FIG. 9 is a cross sectional view taken on line IX-IX in FIG. 8. An integrated circuit 5 is soldered to a back side of a printed circuit board 4 a front side of which has microswitches 3 mounted thereon, which are turned on and off by push buttons 2 of a portable telephone set 1. With an advance of a micro-fabrication technique, the integrated circuit 5 of a BGA package type has come to be widely used, recently. The integrated circuit 5 of the BGA package type is constructed such that a number of ball-like terminals are arrayed in a grid fashion on a bottom surface, and those ball-like terminals are soldered onto corresponding positions on the printed circuit board 4, respectively. Also for the printed circuit board 4 which is assembled into the small portable electronic device, the printed circuit board of the thin type used in recent years more and more increases its number.
When a telephone number is entered to the portable telephone set 1, the push buttons 2 are successively depressed to turn on the microswitches 3. At this time, a force of the finger pressing down each button is transmitted to the printed circuit board 4, so that the thin printed circuit board 4 is somewhat deformed. However, the soldering parts are not peeled off by such a force level as of the operation of entering the telephone number since the back side of the BGA package 5 is soldered onto the printed circuit board 4 over a broad area.
Recently, the portable telephone set of the type which allows the user to play a game during a queuing time of the telephone set increases its number, however. In playing the game, a navigation key for moving the cursor is in heavy usage. Through frequent and repeated button depressions, stress is accumulated in the soldering parts of the BGA package 5, which is soldered to the printed circuit board on the back side of the navigation key. As a result, peeling-off of the soldering part will highly possibly occur.
The soldering parts are located on the back side of the BGA package 5. Therefore, when the soldering part is peeled off, it is difficult to repair the peeled soldering part by re-soldering. Accordingly, the whole printed circuit board 4 must be replaced with a new one. This creates a problem of increasing the repairing cost. Such a low reliability of the soldering parts results in low reliability of the portable telephone set. The same problem arises in general portable electronic devices which each allow the user to play a game, as well as the portable telephone set.